Tailorable Nanoporous Hydroxyapatite Scaffolds for Electrothermal Catalysis

Polarized hydroxyapatite (HAp) scaffolds with customized architecture at the nanoscale have been presented as a green alternative to conventional catalysts used for carbon and dinitrogen fixation. HAp printable inks with controlled nanoporosity and rheological properties have been successfully achieved by incorporating Pluronic hydrogel. Nanoporous scaffolds with good mechanical properties, as demonstrated by means of the nanoindentation technique, have been obtained by a sintering treatment and the posterior thermally induced polarization process. Their catalytic activity has been evaluated by considering three different key reactions (all in the presence of liquid water): (1) the synthesis of amino acids from gas mixtures of N2, CO2, and CH4; (2) the production of ethanol from gas mixtures of CO2 and CH4; and (3) the synthesis of ammonia from N2 gas. Comparison of the yields obtained by using nanoporous and nonporous (conventional) polarized HAp catalysts shows that both the nanoporosity and water absorption capacity of the former represent a drawback when the catalytic reaction requires auxiliary coating layers, as for example for the production of amino acids. This is because the surface nanopores achieved by incorporating Pluronic hydrogel are completely hindered by such auxiliary coating layers. On the contrary, the catalytic activity improves drastically for reactions in which the HAp-based scaffolds with enhanced nanoporosity are used as catalysts. More specifically, the carbon fixation from CO2 and CH4 to yield ethanol improves by more than 3000% when compared with nonporous HAp catalyst. Similarly, the synthesis of ammonia by dinitrogen fixation increases by more than 2000%. Therefore, HAp catalysts based on nanoporous scaffolds exhibit an extraordinary potential for scalability and industrial utilization for many chemical reactions, enabling a feasible green chemistry alternative to catalysts based on heavy metals.


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Description S2-S6 Experimental methods. S6 Scheme S1. Geometrical comparison of 60-HAp/c cubes and HAp/c disks. S7 Table S1. Water absorption capability. Table S2. Macroscopic parameters of the 60-HAp/c and HAp/c catalysts. Table S3. Coating conditions for the synthesis of amino acids. S8 Figure S1. Pore size distribution diagrams. S8 Figure S2. Photographs corresponding to different discarded HAp inks. S9 Figure S3. Representative water absorption capability photographs. S10 Figure S4. SEM micrographs corresponding to the samples listed in Table  S1. S11 Figure S5. Comparison of the ammonium yields obtained from the N 2 fixation reaction as catalyzed by HAp/c and 60-HAp/c.

Synthesis of Hydroxyapatite (HAp)
15 mL of 0.5 M (NH 4 ) 2 HPO 4 in de-ionized water were added at a rate of 2 mL/min to 25 mL of a 0.5 M of Ca(NO 3 ) 2 solution in ethanol with pH previously adjusted to 11 using ammonium hydroxide solution. The mixture was left aging for 1 h under gentle agitation (150 rpm) at room temperature. Hydrothermal treatment at 150 ºC was applied using an autoclave Digestec DAB-2 for 24 h. The autoclave was allowed to cool down before opening. The precipitates were separated by centrifugation and washed with water and a 60/40 v/v mixture of ethanol/water (twice). After freeze-drying it for 3 days, a white powder was obtained.

Synthesis of Pluronic ® F-127 hydrogel
S3 25 g of distilled water was mixed with 25 g of Pluronic® F-127 polymer using a FlackTek SpeedMixer at 3500 rpm for 5 minutes. After that, 50 g of Pluronic® polymer were added and vigorously stirred using the same conditions. The resultant hydrogel was stored at 4 ºC.

Characterization
Structural analyses were performed by means of an inVia Qontor confocal Raman microscope (Renishaw) equipped with a Renishaw Centrus 2957T2 detector and a 532 nm laser. In order to obtain representative data, 32 single point spectra were averaged.
Wide angle X-ray diffraction (WAXD) studies were conducted using a Brucker D8 Advance model with Bragg-Brentano 2 configuration and Cu K α radiation (λ = 0.1542 nm).
Measurements were performed in a 2 range of 20º-60º in steps of 0.02º and scan speed of 2 s, using a one-dimensional Lynx Eye detector. The crystallinity ( c ) was obtained using the following expression: (S1) 300 112/300 is the intensity of the (300) reflection and is the intensity of the hollow I 300 V 112/300 between the (112) and (300) reflections, which disappears in non-crystalline samples. The crystallite size, L hkl , was calculated using the Debye-Scherrer equation where  is the wavelength of the monochromatic X-ray beam, B is the full width at half maximum of the peak at the maximum intensity, and  hkl is the peak diffraction angle that satisfies the Bragg's law for the (hkl) plane.

S4
SEM studies were carried out using a Focused Ion Beam Zeiss Neon40 microscope operating at 5 kV equipped with an EDX (20 kV) spectroscopy system. The latter technique was used to estimate the composition of the distribution of the ATMP/ZC/ATMP coatings.
Water absorption capability was computed by means of a contact angle measuring equipment OCA 15EC (Data-Physics Instruments). To dispense the water droplet, a 500 µL DS500/GT glass syringe and a needle SNS 021/011 were used. The absorption flow rates were calculated by dropping 1.5 µL water droplets onto the surface of the samples while recording with a 30 fps camera. Then, the photograms were analyzed.

Synthesis of Amino Acids
The catalytic synthesis of glycine (Gly) and alanine (Ala) by means of carbon and dinitrogen fixation were prepared using HAp/c or 60-HAp/c catalysts, which were coated with two layers of ATMP separated by an intermediate layer of ZC (i.e. ATMP/ZC/ATMP).
ATMP and ZC layers were prepared by depositing 100 µL of the corresponding solutions.
Different concentrations of each coating solution, which are listed in Table S1, were tested.
After deposition of each coating solution, samples were dried at room temperature for at least 8 h before the deposition of the next layer.
The reactor consisted in an inert reaction chamber coated with a perfluorinated polymer (120 mL), in which both the catalyst and water (1 mL) were incorporated. The reactor was equipped with an inlet valve for the entrance of N 2 , CH 4 , CO 2 and an outlet valve to recover the gaseous reaction products. A UV lamp (GPH265T5L/4, 253.7 nm) was also placed in the middle of the reactor to irradiate the catalyst directly, the lamp being protected by a UV transparent quartz tube. All surfaces were coated with a thin film of a perfluorinated polymer S5 in order to avoid any contact between the reaction medium and the reactor surfaces, in this way discarding other catalyst effects.
Reactions were performed at 95 ºC for a reaction time of 48 h. Both HAp/c disks and 60-HAp/c cubes coated with ATMP/ZC/ATMP and 1 mL of de-ionized liquid water were initially incorporated into the reaction chamber (reactions were performed separately for each catalyst). The chamber was extensively purged with the first selected gas (N 2 ) in order to eliminate the initial air content. Each selected gas was introduced to increase the reaction chamber pressure (measured at room temperature) to the target pressure. In all cases the chamber pressure was increased up to 6 bar by introducing sequentially 2 bar of each feeding reaction gas (i.e. N 2 , CH 4 and CO 2 ).

Synthesis of ethanol
The reaction was performed using the same reactor chamber under a CO 2 and CH 4 atmosphere (3 bar each), at 140 ºC but without the presence of UV light for 48 h. Initially, CO 2 was used to purge the reactor. The catalyst, HAp/c disks or 60-HAp/c cubes without any coating, and 1 mL of de-ionized liquid water were incorporated into the reaction chamber (reactions were performed separately for each catalyst). Additionally, the effect of the initial water content on the reaction yield was investigated for one of the 60-HAp/c catalysts.

Quantitative analyses
The reaction products were analyzed by 1 H-NMR spectroscopy. All 1 H-NMR spectra were acquired with a Bruker Avance III-400 spectrometer operating at 400.1 MHz. The chemical shift was calibrated using tetramethylsilane as internal standard. Sixty-four scans were recorded in all cases. In order to remove the products formed on the catalysts from reactions S6 involving CO 2 and CH 4 , samples were dissolved in deuterated water containing 100 mM of HCl and 50 mM of NaCl with the final addition of deuterated water.
In the case of the dinitrogen fixation reaction to produce ammonia, the catalyst (10 mg) was dissolved in 15 mL of water with pH adjusted to 2.1  0.2 using 7.6 mM H 2 SO 4 , to promote the conversion of ammonia in NH 4 + , and applying 4 cycles that involved sonication   Table S2. Macroscopic parameters of the 60-HAp/c and HAp/c catalysts (Scheme S1). The exposed area only considers the faces of the geometry that are exposed in the reaction.   S10 Figure S4. SEM micrographs of coated 60-HAp/c catalysts listed in Table S1. The nomenclature indicates the concentration (in mM) used for the different ATMP and ZC layers. S11 Figure S5. Comparison of the ammonium yields obtained from the N 2 fixation reaction as catalyzed by HAp/c and 60-HAp/c. Reactions were performed using an initial N 2 pressure (6 bar), 20 mL of water, at 120 ºC with UV irradiation for 24 h.